What's a non-roller cam equivilant to an E303

The sound comes from the duration and valve event timing so look around at the specs and find one similar.
You will find that the lobes are very different on a flat tappet and a roller tappet cam even if the specs are the same. Cams with the same specs will sound less agressive in a flat tappet engine in general because the valve opens and closes slower (the lobe may have the same height and duration but is still physically smaller since the 'ramp' to max lift is more gentle).
In addition to that, the engine size, head design, and valve size have a huge influence on the idle. Finally, larger displacement engines can take more cam and still have a reasonable idle.
I assume you're building a 289 for your '66 (?) If so, the stock firing order is different from the late model cars (351W order, 5.0 HO, etc). You may want to get a cam that has the 351 firing order to get the sound at the tailpipe to sound similar to a 5.0 HO. You would then just install the plug wires accordingly....
I have the Edelbrock 7122 in my newly rebuilt 302 but it has not been fired up yet so I can't make any recommendataions about it.... The Comp cam in my 5.0 has a seriouos kick butt idle though (see sig.).
To be honest, I think the early v8's have their own sound and I doubt that you will be able to reproduce the 5.0 sound unless you actually use a late model fuel injected engine.

The "sound" you think distinguishes a late model 5.0 from an early small block doesn't come from the different firing order. It's a result of a completely different exhaust system. If you take a late model 5.0 and swap cams to one that's got the earlier firing order, you will never notice the difference in the sound. A good flat tappet cam for a 289 to get the "lope" will need arond a .500-525 lift and a 280-290 degree duration. Hard to equate a roller cam profile to a flat tappet cam as oz has posted before.

E cams don't have crazy ramps on them. A hyd flat tappet with around 220@ .050 lift will give you that lope. Try to match the cam with the rest of the motor. The wrong cam will cost hp, trq and drivability. Get a cam company to help you out.

the e303 cam specs are as follows .498 lift w/1:6 rockers, 220 degrees duration at .050 282 advertised, single pattern, 110 lobe seperation. a flat tapped of simalar specs will sound simalar just a little smoother in tone.

the first time i bought an aftermarket roler cam the first thing that came to mind when i oppened the box was WTF they sent me a circle track cam, they just look way bigger than they are.